1. Field of the Invention
This invention relates to an electroluminescent silicon device.
2. Discussion of Prior Art
There is considerable interest in the production of silicon-based or silicon-compatible light emitting devices for use in opto-electronic integrated circuits. In the course of work to develop such devices, visible electroluminescence has been observed from silicon in a variety of circumstances some of which are described briefly below.
R. Newman in "Visible Light from Silicon p-n Junction", Phys. Rev. Vol 100, p700 (1955) and Chynoweth and McKay in "Photon Emission from Avalanche Breakdown in Silicon", Phys. Rev., Vol 102, P369 (1956) describe very broad band electroluminescent emission when bulk silicon p-n junctions are reverse biased to "avalanche breakdown". The emission extends from about 3 eV to below 1 eV and is of very low efficiency, typically 10.sup.-5 photons per electron.
Visible electroluminescence has also been observed from metal-silicon dioxide-silicon structures in which the silicon dioxide layer contains very small silicon crystallites (less than 50 .ANG. wide). This is described by Di Maria et al in "Electroluminescence Studies in Silicon Dioxide Films Containing Tiny Silicon Islands", J. Appl. Phys., Vol 56, p401 (1984). Again the emission has very low efficiency, typically less than 10.sup.-6 photons per electron.
W. Waring and E. A. Benjamin in "Luminesence during Anodic Oxidation of Silicon", J. Electrochem. Soc., Vol 11, p1256 (1964) describe visible electroluminescence from bulk silicon surfaces when they are anodically biased in electrolytes capable of oxidising the surface. Once again the efficiency of the emission is low, and in addition it is unstable since it results from chemical changes at the relevant interface. More recent work in this field is described by A P Baraban et al in "Electroluminescence Spectra in the System Silicon-Silicon Dioxide-Electrolyte", Soviet Electrochem., Vol 20, p507 (1984).
A. Gee in "Electrochemiluminescence at a Silicon Anode in Contact with an Electrolyte", J. Electrochem. Soc., Vol 107, p787 (1960) describes visible electroluminescence from chemically produced stain films on bulk p-type silicon under anodic bias in many electrolytes. The emission of the light results from the anodic oxidation of the film and is irreversible.
The above approaches to obtaining electroluminescence from silicon therefore have a number of drawbacks, primarily very low efficiency or irreversibility. These drawbacks render the approaches unsuitable for producing practical electroluminescent silicon devices.
Recently L. T. Canham in "Silicon Quantum Wire Array Fabrication by Electrochemical and Chemical Dissolution of Wafers", Appl. Phys. Lett., Vol 57, p1046, (1990), describes the fabrication of free standing silicon quantum wire arrays in surface layers of bulk silicon wafers. Free standing wires are taken to be wires which are not immersed or embedded in another material. The wires themselves need not be isolated columns but may be interlinked. Quantum wires are wires in which the diameter is sufficiently small such that quantum confinement of carriers occurs within them. The result of this is that the bandgap of the material concerned, in this case silicon, is significantly increased. Visible red photoluminescence is emitted from these quantum wire arrays under appropriate irradiation. These arrays are however highly resistive and therefore unlikely to yield significant electroluminescence at the low applied bias voltages required for conventional integrated circuitry. This work has been reproduced elsewhere, and workers have gone on to suggest that appropriately doped free standing silicon quantum wires would provide an array of lower resistivity capable of supporting appreciable current and exhibiting electroluminescence. Despite this, there has been no report of silicon quantum wires which are doped appropriately for the above electroluminescent behaviour to be exhibited.